The following U.S. Patents are incorporated herein by reference in entirety.
U.S. Pat. No. 7,387,556 discloses an exhaust system for a marine propulsion device, which directs a flow of exhaust gas from an engine located within the marine vessel, and preferably within a bilge portion of the marine vessel, through a housing which is rotatable and supported below the marine vessel. The exhaust passageway extends through an interface between stationary and rotatable portions of the marine propulsion device, through a cavity formed in the housing, and outwardly through hubs of pusher propellers to conduct the exhaust gas away from the propellers without causing a deleterious condition referred to as ventilation.
U.S. Pat. No. 5,954,554 discloses an outboard drive that involves an improved exhaust system that increases the reverse thrust produced by the outboard drive. The exhaust system includes a first exhaust passage and a second exhaust passage that stems from a first exhaust passage. A flow control device operates within the exhaust system to control exhaust gas flow through second passage depending upon the drive condition (either forward or reverse) of the outboard drive. The flow control device permits exhaust gas flow through the second passage when the outboard drive operates in reverse, while inhibiting exhaust gas flow through the second passage when the outboard drive operates under a forward drive condition. In this manner, the improved exhaust system reduces exhaust gas back pressure and thrust degradation due to exhaust gas entrainment in the propeller when the outboard drive operates in reverse.
U.S. Pat. No. 5,759,073 discloses a propulsion system for a marine drive, which includes a pair of counter-rotating propellers, and provides improved acceleration from idle or low speeds. Engine exhaust from an engine which powers the marine drive is conveyed to the water about each of the propellers. The exhaust gases aerate the water about each propeller to reduce drag resistance on each propeller. Several embodiments of the propulsion system are disclosed which convey the exhaust gases to both propellers for this purpose.
U.S. Pat. No. 5,299,961 discloses a marine propulsion unit that includes a cavitation cavity defining a supplemental exhaust gas passage terminating at an exhaust outlet port which is adapted to be opened and closed by a normally closed flapper control valve member. The exhaust outlet port includes lateral sidewalls and the flapper control valve member includes a fixed portion secured to one of the lateral sidewalls and a cantilevered portion that extends across the exhaust outlet port. The cantilevered portion is adapted to flex relative to the fixed portion so as to open and close the exhaust outlet port in response to dynamic forces acting on the control valve member during operation of the watercraft. A stopper plate is provided to limit the deflection of the cantilevered portion of control valve member. The cantilevered portion of the control valve member is further provided with a fin which projects into the water during operation of the watercraft so that the dynamic pressure of the water acts against the fin the supplement the dynamic pressure exerted on the control valve member by the exhaust gas pressure from within the supplemental exhaust passage and to cause the control valve to open. When the watercraft is stopped, running at low speeds, decelerating or immediately after planning, the control valve will assume a position in which it closes the supplemental exhaust outlet port to reduce exhaust noise.